Reduced distortion driving of balanced transmission line phase bridge discriminators

ABSTRACT

A balanced transmission line phase bridge frequency discriminator of conventional type is driven by a circuit which reduces even order harmonic content of a frequency modulated radio signal, as seen by said discriminator, essentially to zero, and which reduces progressively the amplitudes of odd order harmonics, if these occur, in order to increase the useful demodulated output of the detector. Well balanced push-pull amplifiers employing bifilar windings to enhance balance are employed in one embodiment, to effect the drive; in another embodiment filtering is resorted to to remove harmonic response; and in still another embodiment both these expedients are employed concurrently.

United States Patent Modafferi 1 Feb. 29, 1972 [54] REDUCED DISTORTION DRIVING 0F 3,217,263 11/1965 Starreveld et al. ..329/l03 X BALANCED TRANSMISSION LINE OTHER PUBLICATIONS PHASE BRIDGE DISCRIMINATORS Hurel et al., Fundamental Frequency Detector" IBM [72] Inventor: Richard Modafiefi Vestal Technical Disclosure Bulletin Vol. ll, No. 5, Oct. 1968, p. [73] Assignee: McIntosh Laboratory, Inc., Binghamton, 490

N.Y. Primary Examiner-Alfred L. Brody [22] Filed: Nov. 21, 1969 An0mey Hm-via & Rose 21 A 1. N0.: 878 768 1 pp 57 ABSTRACT 52 us. or. ..329/192 325/323 325/477 A balanced "ansmissm" Phase bn'dge frequency 328/l65 329/50 329/156 329/1 330/15 criminator of conventional type is driven by a circuit which [5i] Int. Cl. H631! 3/00 reduces even order harmonic content of a frequency modu- [58] Field of Search "329/ 136 lated radio signal, as seen by said discriminator, essentially to 325/477 5 328/165 6 6 zero, and which reduces progressively the amplitudes of odd order harmonics, if these occur, in order to increase the useful [56] References Cited demodulated output of the detector. Well balanced push-pull amplifiers employing bifilar windings to enhance balance are UNITED STATES PATENTS employed in one embodiment, to effect the drive; in another embodiment filtering is resorted to to remove harmonic 3,028,487 4/1962 Losee ..325/323 response; and in Still another embodiment h these ex i) ii pedients are employed concurrentlyv 3,462,694 8/1969 Avins ..329/1 10 7 Claims, 11 Drawing Figures 62 1 68 7 10 -nnpumen HLTER a 3ST?! \NPUT i PATENTED FEB 2 9 I972 SHEET 2 OF 3 8.5 EOOZMO EwPJ E uwimouwza .50 m mwfi m 8 INVENTOR mcHnRD MODEM-PERI ATTORNEYS Saba PATENTEDFEB29 m2 SHEET 3 [IF 3 INVENTOR R ICHHRD MDDAFFERI ATTORNEYS REDUCED DISTORTION DRIVING OF BALANCED TRANSMISSION LINE PHASE BRIDGE DISCRIMINATORS BACKGROUND OF THE INVENTION The balanced transmission line phase bridge discriminator for detecting FM signals in FM radio receivers (hereinafter called bridge discriminator), and the theory thereof, have been known for many years. See Panter, Modulation, Noise, and Spectral Analysis, McGraw-I-Iill, 1965 p. 410, FIG. l3-3b. However, practical applications for this circuit have been limited because the theoretical performance indicated for the bridge discriminator could not, before the present invention, be achieved in practice. The reason for the defect has been, heretofore, unexplained, and therefore the cure of the defect could not be achieved.

The conventional theory of the discriminator assumes a sinusoidal input signal to the detector. In actual practice, even order harmonics may be generated by circuits driving the discriminator, and these are shown to introduce a wide discrepancy between the theoretical and actual performance exhibited by the bridge discriminator, being the primary cause of distortion. It is toward the elimination of this discrepancy that the present invention is directed, the contribution of the inventor residing in his discovery of the reason for the distortion, suitable devices for reducing distortion, once its mechanism has been discovered, being of a variety of types.

SUMMARY OF THE INVENTION The present invention relates to a method and means for driving the bridge discriminator in a manner which minimizes the distortion produced in the demodulated FM signal and thus in a manner which minimizes the discrepancy between the theoretical and actual performance of the bridge discriminator, by eliminating even order harmonics from the input to the discriminator, and by assuring that odd order harmonies, if they occur, are properly relaxed in amplitudes, the higher order harmonics having the smaller amplitudes. The invention resides primarily in the discovery of the source and mechanism of the distortion.

In investigating the problem of driving the bridge discriminator, it was empirically determined that an input RF waveform containing only odd-order harmonics (fundamental, 3rd, th, etc.), of proper relative amplitudes, like the pure sinusoid, produces minimum distortion in the demodulated FM signal. The present invention deals with the method of feeding and means for feeding the bridge discriminator in such a manner so as to produce no even-order harmonics in the RF waveform. Thus, the present invention relates to a method and means for minimizing distortion in the demodulated FM signal.

In accordance with one embodiment of the present invention, distortion is diminished in the demodulated output signal by feeding the bridge discriminator with an FM input signal which is first amplified and then filtered to remove harmonics. For large deviation ratios, the filter concept works well as it is relatively easy to obtain a straight line response over much of the fundamental transfer characteristic.

Another manner of driving the bridge discriminator so that distortion in the demodulated output signal is negligible is to voltage amplify the FM input signal and then pass the amplified signal through a balanced push-pull power amplifier and a balanced bifilar transformer before feeding same to the bridge discriminator. By so feeding the bridge discriminator, no even-order harmonics are produced in the amplification process and the rolloff of the odd-order harmonics which may be produced by the amplifiers, is at a fast-rate so that a useful and distortion-free output is obtained from the discriminator. This controlled rolloff concept works particularly well where the FM deviation ratio is small.

In a third embodiment of the invention, both of the above teachings are combined.

discriminator to attain a demodulated output signal having negligible distortion.

These and other objects of the invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following descriptions taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of a balanced transmission line phase bridge discriminator;

FIG. 2 is a plot of the output voltage versus the input frequency of an ideal bridge discriminator driven by an ideal RF frequency modulated sinusoidal waveform;

FIG. 3 is a plot of the output voltage versus the input frequency of an ideal bridge discriminator driven by a nonsinusoidal input signal;

FIG. 4 is a block diagram of a first embodiment of the invention employing the filter concept of the present invention;

FIG. 5 is a plot of a transfer characteristic of the bridge discriminator circuit shown in FIG. 4;

FIG. 6 is a block diagram of a further embodiment of the invention employing the controlled rolloff concept of the present invention;

FIG. 7 is a plot of a transfer characteristic of the bridge discriminator circuit shown in FIG. 6;

FIG. 8 is a block diagram of a bridge discriminator drive circuit combining the concepts utilized in FIG. 4 and FIG. 6;

FIG. 9 is a schematic circuit diagram of the configuration depicted in block diagram form in FIG. 4;

FIG. I0 is a schematic circuit diagram of the configuration depicted in block diagram form in FIG. 6; and

FIG. 11 is a schematic circuit diagram similar to FIG. 10, employing input and output balanced bifilar transformers, in an IF amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, the circuit diagram of a balanced transmission line phase bridge discriminator for the IF frequency signal is an FM broadcast receiver is shown generally at 10. The bridge discriminator 10 comprises a first transmission line 12, one-eighth A long at 10.7 me. the aforesaid IF which is opemcircuited and a second transmission line 14, one-eighth A long, at that same frequency, which is short-circuited. Forming a Wheatstone bridge in combination with the two transmission line sections 12 and 14 are equal resistors l6 and 18, respectively, whose resistance is equal to the nominal impedance (Z,) of the transmission line. The F M input signal is fed into terminals 20 and22, terminal 20 serving to feed the transmission lines 12 and 14 through respective resistors 16 and 18 and terminal 22 being connected to ground. As the frequency of the IF varies the impedances of the two transmission lines vary in opposite senses.

One terminal of a capacitor 24 is connected to the junction of transmission line section 12 and resistor 16, and the other terminal of capacitor 24 is connected-to a common junction between a pair of diodes 28 and 30. The remaining tenninalof diode 28 isgrounded and the remaining terminal of diode 30 is connected to'one terminal of a capacitor 26, the other terminal of-which is connected to ground. Similarly arranged, but associated with transmission line section 14 and resistor 18, are a pair of capacitors 32 and 34 and a pair ofdiodes 36 and 38.

Capacitors 24 and 26 and diodes 28 and 30form a voltagedoubling peak rectifier referred to generally as 40. Similarly, capacitors 32 and 34 diodes 36 and 38 form another voltagedoubling peak-rectifier designated generally at 42. The respeclive voltage-doubling peak rectifiers 40 and 42 are thus AM detectors, frequency to amplitude discrimination occurring in the bridge circuit consisting of lines 12 and M and resistors 16 and 18.

A load resistor 46 receives the load currents from the respective AM detectors 40 and d2. One terminal of the load resistor $6 is connected to ground and the other terminal is connected to an adjustable tap 48 on a balancing resistor 50, which resistor 50 serves to balance the load currents from detectors 40 and 42 which pass through the load resistor 46. The position of the tap 48 is initially set to yield a zero DC voltage across the load resistor 66 when the bridge discriminator I is driven by an input signal at the center frequency of the discriminator,

An RF low-pass filter is defined by a capacitor 52, and inductor S4 and a second capacitor 56. This filter is set to remove from the demodulated output any residual voltage at or near the center frequency in the output of the phase discriminator 10, so that the output signal at terminal 58 is an audio signal.

The phase discriminator 10, shown in FIG. I, operates as follows. An FM input signal is impressed upon terminals 26 and 22 and is fed through the Wheatstone bridge defined by transmission line sections 12 and 14 and resistors 16 and 18. An RF voltage e, appears between transmission line section 12 and resistor 16, and an RF voltage e appears between transmission line section 14 and resistor 18. Voltages e, and 2 vary with frequency in such a manner that their RMS arithmetic difference (e -e is a straight line function. RF voltage e is peak rectified by AM detector 40 and RF voltage 2 is peak rectified by AM detector 42. Since the respective AM detectors 40 and 42 are connected to balancing resistor Si in opposite polarities, the voltage developed across the load resistor 46 is the arithmetic difference of the currents in the load resistor due to each AM detector. Thus, the voltage developed across the load resistor 46 is in direct proportion to the true arithmetic difference of the RF voltages e and e Hence, ideally, the demodulated output appearing at output terminals 58 and 60 is distortionless.

Above, the balanced transmission line phase bridge discriminator circuit has been described. It has been noted that ideally, the RF waveform with which to drive the bridge discriminator should be a sinusoid. The importance of a pure sinusoidal driving signal is clearly demonstrated by inspecting the transfer characteristics of the bridge discriminator shown in FIG. 1, for both sinusoidal and nonsinusoidal input signals. (It should here be noted that while, strictly, an FM wave is not a sinusoid, due to modulation, the assumption of a sinusoidal wave shape over short periods of time is valid if the modulating frequency is less by several orders of magnitude than the carrier frequency).

Assuming first a sinusoidal driving signal, the output of an ideal bridge discriminator is defined by the following equation:

an: M2 an. i r el :01-

wherein WC is the center frequency of the discriminator corresponding with the unmodulated value of the input FM signal. If the output voltage defined in equation I is plotted with respect to the variable In, a series of straight line segments result. This plot of output voltage versus frequency is the transfer characteristic of the bridge discriminator and is shown in FIG. 2. It will be noted in H6. 2 that the center frequency of the bridge discriminator 300 is 10.7 MHz, and, since a pure sinusoid is assumed, no harmonics are present in the input signal.

As noted above, an FM sinusoid is difficult to amplify without distortion, since a transmission line bridge discriminator requires power in its drive signal. It will be assumed that a nonsinusoidal input drives the bridge discriminator. With a generalized nonsinusoidal input signal, the output of the balanced transmission line phase bridge discriminator is as follows:

When the output voltage, defined in equation 2, is plotted against the variable in, the. transfer characteristic illustrated in FIG. 3 results.

In FIG. 3, it will be noted that the even-order harmonics, K K K swing about the corners of the idealized transfer characteristic. These corner voltages produce severe evenorder harmonic distortion in the demodulated FM output. In FIG. 3, it will also be seen that the odd-order harmonics, IQ, K K in the FM input, fall always on a linear portion of the idealized transfer characteristic during frequency modula tion and hence, do not cause distortion, but the response to the third harmonic tends to cancel the response to the fundamental.

Since the voltage K opposes K it will tend to reduce the demodulated output. It can be seen that if the input to the bridge discriminator is a square wave, no demodulated output will occur. In summary then, the method of driving the bridge discriminator must take into consideration the following ideas:

1. Ideal case: Purely sinusoidal input FM signal 2. Nonideal case: (a) Effectively complete suppression of even-order harmonics in FM signal; (b) rollotf of the oddorder coefficients (K K K at a rate faster than a square wave, so that useful output is obtained.

From FIG. 3, it should be evident that when a balanced transmission line phase bridge discriminator is driven by a nonsinusoidal driving signal, the even-order harmonics of the driving signal introduce severe distortion. Thus, for the bridge discriminator to be practical for high-quality reception, it is essential that even-order harmonics be suppressed or not produced in circuitry antecedent to the discriminator. It is also required that odd order harmonics be either suppressed or suitably attenuated so that they do not introduce undue attenuation when combined with the response of the fundamental.

With reference now to FIG. 4, one embodiment of the present invention will be described. In FIG. 4, there is shown a block diagram of a circuit for feeding a balanced transmission line phase bridge discriminator with a nonsinusoidal FM input signal in a manner which results in a relatively distortion-free demodulated output. An FM input signal having its midfrequency at the midfrequency of the discriminator is fed to the circuit by means of input terminals 62 and 64. The FM input signal is first amplified by an amplifier 66 and is then filtered by a filter 68. The amplifier 66 may be any type of amplifying device, but the filter 68 must be designed to remove all harmonics generated by amplifier 66, or in circuits antecedent thereto.

The filter 68 generates a frequency modulated but essentially sinusoidal RF output signal for driving the bridge discriminator. The filter should have a nominally fiat-amplitude and linear-phase shift as a function of frequency, within its pass band. The filter 68 thus serves to remove all the harmonies from the FM input signal fed to terminals 62 and 64 and, as a consequence, feeds the bridge discriminator 10 with a driving signal containing only the fundamental. The demodulated signal is removed from output terminals 70 and 72.

In FIG. 5, the transfer characteristic of the discriminator shown in FIG. d is depicted. The output signal appearing across terminals 70 and 72 of FIG. 4 is here plotted against frequency. Frequencies 74 and 76 represent the band of frequencies passed by the filter 68. The linear portion of the characteristic curve is centered between frequencies 74 and 76 and, in this manner, the discriminator l0 sees" only the fundamental of the FM input signal. The harmonics, both odd and even-order, are far removed from the band of frequencies passed by the filter 68.

Above, there has been described a method for feeding a bridge discriminator so that the output of the discriminator is relatively distortionless. This method involves removing harmonies from the modulated input signal before the signal reaches the bridge discriminator, and a filter is used to accomplish this desirable result. The concept described above is par ticularly useful if the FM deviation ratio is large (i.e., over a large portion of the overall transfer characteristic). This is true, since, with a filter, it is quite easy to obtain an all pass characteristic over a large portion of the fundamental transfer characteristic.

If the FM deviation ratio is small, however, it is contemplated by the present invention that a circuit more suitable than that described above be employed. There follows a description of a circuit useful for feeding a bridge discriminator when the FM deviation ratio is small.

Referring again to FIG. 3, it will be remembered that the even-order harmonics fall on the corners of the idealized transfer characteristic and are therefore troublesome. The odd-order harmonics fall on linear portions of the idealized transfer characteristic and, therefore, do not cause distortion. However, since, for example, the voltage represented by K opposes the voltage represented by K,, the demodulated output will be reduced. It can be shown that if the input to the bridge discriminator is a square wave, no demodulated output will occur. The present invention therefore contemplates that the odd-order coefiicients (K K K be made to roll off at a rate faster than a square wave so that a useful and maximum output can be obtained.

FIG. 6 is a block diagram of a circuit for controlling the rolloff of the odd-order coefficients associated with a nonsinusoidal FM input signal and, further, for eliminating evenorder harmonics. An FM input signal is applied to terminals 78 and 80. The input signal is voltage amplified by an amplifier 82 and is then fed to a push-pull power amplifier 84. The push-pull amplifier 84 provides no even-order harmonics at its output in response to the FM input impressed upon terminals 78 and 80. The output circuit of the pushpull amplifier 84 is a balanced bifilar transformer 86, which is sufficiently frequency sensitive in its response to provide the required rolloff. The output of the bifilar transformer 86 drives the bridge discriminator 10 and a demodulated output signal is extracted from the circuit by means of output terminals 88 and 90.

In FIG. 7, the transfer characteristic of the circuit illustrated in FIG. 6 is depicted. It should first be noted that there are no even-order harmonics. This is due to the operation of the push-pull power amplifier 84. Also, the characteristic curve diminishes in amplitude with increasing frequency due to the design of the transformer. In this manner, the effect of K on the fundamental represented by K, is diminished. The balanced bifilar output transformer 86 causes the abovedescribed rolloff, and it has been found that the best performance results when each primary half consists of a transmission line pair whose characteristic impedance is given by 2,, M R I-Iowever, windings with characteristic impedances from one-third to three times the above value operate well.

Above, there have been described two embodiments of the present invention, one employing a filter to feed a bridge discriminator and the other employing the combination of a push-pull power amplifier and a balanced bifilar transformer to feed a bridge discriminator. The first embodiment of the invention, the filter concept, effectively serves to suppress all harmonics in an FM signal and is most useful if the FM deviation ratio is large. The second embodiment, the balanced amplifier rolloff concept, produces no even harmonics, controls the rolloff of the odd harmonics and serves most effectively when the FM deviation ratio is small.

In a third embodiment of the present invention, the advantages of the first and second embodiments are combined. With reference, now, to FIG. 8, the third embodiment of the present invention will be described. A modulated FM input signal is fed to the circuit at input terminals 92 and 94. First, the input signal is power amplified by an amplifier 96 and is then fed to a balanced bifilar transformer 98. The output from the transformer 93 is filtered by a filter I00 and is ultimately fed to the bridge discriminator 10. The demodulated output signal, which is relatively distortion-free, is removed from the circuit at output terminals 102 and 104. It will be noted in FIG. 8 that the push-pull power amplifier and balanced bifilar transformer of FIG. 6 are combined with the filter of FIG. 4. Thus, the circuit of FIG. 8 combines the operating characteristics of FIGS. 4 and 6, as well as the advantages of each.

Now, specific examples of the present invention will be described. Referring first to FIG. F, a circuit schematic of the filter concept of FIG. 4 is illustrated. An FM signal generator having a center frequency of, for example, 10.7 MHz, is shown at 106. The numeral 66 represents an amplifier which, in this Figure, is of the vacuum tube variety. The output circuit of the amplifier 66 is a filter 68.

Filter 68 comprises a shunt capacitor 108 which represents the tube capacitance and the stray circuit capacitance. Forming a series path between the amplifier 66' and the bridge discriminator 10, is a variable inductor 110. A capacitor 112 is connected between the forward end of the inductor and ground. The filter 68' is tuned to pass 10.7 MHZ when the FM signal generator 106 is set to 10.7 MHz, the variable inductor 1 10 serving to tune the filter, but to reject harmonics.

A coupling capacitor 114 connects the filter 68' to the bridge discriminator 10, the discriminator 10 being a duplicate of that discriminator illustrated in FIG. 1. Output terminals 116 and 118 serve to remove the demodulated output from the circuit.

With reference now to FIG. 10, there appears a circuit schematic of the bridge discriminator illustrated, in block, in FIG. 6. An FM signal generator tuned to 10.7 MHz is shown at 120. This signal generator feeds a power amplifier shown generally at 122. Amplifier 122 may be of the type MC-I350," an integrated circuit amplifier made by Motorola. The amplifier 122 feeds the bases of a pair of transistors 124 and 126, arranged in push-pull, while the respective bases of transistors 124 and 126 receive a biasing potential through a variable resistor 128 associated with a pair of resistors 130 and 132. The function of resistor 128 is described below.

A balanced bifilar transformer is shown generally at 134. The primary winding of the transformer 134 is split into two sections, 136 and 138, respectively. Similarly, the secondary winding of the transformer 134 is split into two sections, 140 and 142, respectively. A common core 144 is provided.

The common junction of the primary windings 136 and 138' are connected to the emitters of transistors 124 and 126 through a pair of RC circuits 146 and 148 and a DC blocking capacitor 150. The ends of the windings 136 and 138, remote from the common junction of these windings, are connected directly to the collectors of transistors 124 and 126, respectively.

An inductor 152 provides an RF choke for a DC supply applied to the common junction of windings I36 and 138. The tap on resistor 128 is adjusted to achieve minimum distortion by equalizing currents to the bases of transistors 124, 126, deriving from bias lead 129.

The end of the winding 140, remote from the common junction of windings I40 and 142, is indicated at 154 and serves to define an output terminal for the unbalanced bifilar transformer. Terminal 154, shown in FIG. 10, is connected to the bridge discriminator 10 at 156, shown in FIG. 9.

In FIG. 10, the amplifier system and the transformers are designed to give the proper amount of controlled response rolloff. Thus, this circuit accepts a nonsinusoidal input, acts upon this input and provides an output signal having no even-order harmonics, and odd order harmonics which rolloff rapidly as the order of the harmonics increases, for use in a bridge discriminator. The output of the bridge discriminator is thus relatively distortion free.

In FIG. 11, there is illustrated an alternate embodiment of the circuit shown in FIG. 10. An FM signal generator 158 feeds an amplifier 160, which may be of the type "CA 3042,"

china: mm-

made by RCA. The amplifier 160, in turn, drives a first balanced bifilar transformer 162, transformer 162 comprising a pair of primary windings 164 and 166 and a pair of secondary windings 168 and 170. The amplifier 160 drives the transformer 162 by means of its primary windings 164 and 166, which are connected in series. A pair of transistors 172 and 174 are connected in push-pull and have their bases fed from the ends of windings 168 and 170, remote from their common junction. The bases of transistors 172 and 174 are biased in a manner dependent upon the setting of a resistor 176. The resistor 176 is variable and is adjusted so that minimum distortion results.

A second balanced bifilar transformer 178 is provided. Transformer 178 comprises a pair of primary windings 180 and 182 and a pair of secondary windings 184 and 186. The common junction between windings 180 and 182 is connected to the emitters of transistors 172 and 174 through a pair of RC circuits 188 and 190 and further through a coupling capacitor 192. The common junction between the primary windings of transformer 178 is further connected to the common junction between the secondary windings of transformer 162 via resistor 194. The ends of the primary windings 180 and 182, remote from the common junction therebetween, are connected, respectively, to the collector junctions of transistors 172 and 174. The junction of winding 184, remote from the common connection between the two secondary windings, is indicated at 196 and serves as an output terminal for the circuit of FIG. 11. Output terminal 196 is connected to the input of the bridge discriminator 10 at 156, illustrated in FIG. 9.

In the embodiment of FIG. 11, a capacitor 198 is provided between the respective collectors of transistors 172 and 174. Capacitor 198 is therefore connected across the primary of the output transformer 178. In this manner, the value of the capacitor 198 may be chosen to result in the proper amount of controlled frequency response rolloff.

Above, there have been described three embodiments of the present invention. Each of these embodiments relate to feeding a balanced transmission line phase bridge discriminator in a manner resulting in a minimum of output distortion. Distortion results since an ideal sinusoidal input signal cannot practically be obtained. It has been found that this distortion, for the most part, is the result of even-order harmonics in the signal driving the bridge discriminator. The three embodiments of the present invention feed a balanced transmission line bridge discriminator with signals devoid of even-order harmonics. In the first embodiment, all but the fundamental is removed by means of a band pass or low-pass filter. In the second embodiment, the even-order harmonics are removed by a push-pull amplifier and the odd-order harmonics are caused to roll off in a manner avoiding a diminished output response. In a third embodiment, the useful characteristics of the first and second embodiments are combined.

it is well known that push-pull amplifiers produce no second harmonic distortion, provided they are balanced. Any unbalance can and does result in even order distortion. Where push-pull amplifiers of complementary symmetry types are employed, great difficulty is encountered in matching the transistors of a pair. It is therefore desirable to employ a pair of transistors of the same conductivity type. The transistors must now be driven in push-pull, and for this purpose a phase inverter is sometimes used. However, phase inverters are difficult to balance, because output impedances of emitter and collector circuits of the two transistors are unequal. A simple Class B push-pull amplifier, suitable for the present application, should therefore resort to input and output transformers. But simple amplifiers operated from and into a transformer are not inherently distortion free. For example, power gain depends on load resistance, and in the present case this is not constant through a cycle because of the varying reactive load placed upon the frequency discriminator bridge by the voltage doublers. Further, crossover distortion occurs because power gain drops at low emitter current, in the common emitter type of amplifier, as well as because of variation of a at low emitter currents, wherein a is the gain of the transistor of the amplifi- It has been found, and it is a feature of the present invention, that utilization of bifilar input and output transformers goes far to reduce the distortions inherent in push-pull amplifiers. Their effect is to introduce unity coupling between the input amplifier and base drive circuitry of the push-pull amplifier, and also to introduce unity coupling between the load and the output winding of the amplifier. A further effect is to reduce essentially to zero any leakage reactance, so that the transistors operate from and into pure resistance loads. This eliminates a possible source of distortion.

The use of bifilar transformers in tube amplifiers is known, but their utility in push-pull transistor amplifiers free of even order harmonics has not been appreciated.

In the practice of the present invention, amplifier 122 of FIG. 10 is a differential phase inverter, conventional per se, with a high degree of inherent self-balance, and therefore can be assumed to have negligible even-order harmonic output. Filter F is a decoupling filter for the signal frequencies. The output circuit of the amplifier, which is required to convey power to the bridge discriminator is a bifilar transformer 13!], having a single ended secondary. All three windings are bifilarly wound with respect to each other, which assures that the secondary is equally coupled to each half of the amplifier, and also reduced crossover distortion to a negligible value.

In FIG. 11, the driver is a single ended amplifier, and primary winding 166 is grounded for AC through large capacitor C The secondary winding L68 is AC grounded by capacitor C and being bifilarly wound with respect to its own halves and the primary winding, must always remain perfectly balanced for AC.

The amplifiers are biased Class A, because this represents the most distortionless mode of operation. AC grounding then simplifies bias problems and any tendency to unbalance is reduced to negligible values by the bifilarity of the input secondary windings. Two input and two output windings are employed, at both input and output, to achieve more complete balance.

The total effect of the balancing expedients employed results in an amplifier with is virtually free of second harmonic distortion, and therefore a distortion free discriminator.

For best results, a filter is used between the power amplifier and the discriminator, which passes fundamental and reduces harmonics. The attenuation required of this filter is obviously reduced radically if practically no even harmonics are produced by the amplifier, and its rollofi' provides the essential odd harmonics attenuation as a function of order of the barmonies.

I claim:

1. A method for achieving distortionless response of a balanced transmission line phase bridge discriminator, comprising the steps of: receiving an FM input signal; so amplifying the FM input signal as to produce essentially zero evenorder harmonics in the amplified output and feeding the so amplified output to a balanced transmission line phase bridge discriminator.

2. A circuit for providing distortionless response of a balanced transmission line phase bridge discriminator, comprising: means for receiving an FM input signal; amplifier means for amplifying the FM input signal; filter means for receiving from said amplifier the amplified FM input signal, said filter being arranged and adapted to issue a signal which is a replica of the FM input signal devoid of harmonics; and means for feeding said replica to the input of said balanced transmission line bridge discriminator.

3. A balanced transmission line phase bridge discriminator system, comprising: means for receiving an FM input signal; means for amplifying said FM input signal; push-pull power amplifier means for receiving said amplified input signal, said push-pull amplifier means being arranged and adapted to transfer said amplified signal without even-order harmonics; and means for feeding the resultant signal to said balanced transmission line bridge discriminator.

4. The circuit as described in claim 3, wherein said means for controlling the frequency response rolloff includes at least one balanced bifilar transformer connected as the output circuit of said push-pull power amplifier.

5. A circuit for driving a balanced transmission line phase bridge discriminator, comprising: means for receiving an FM input signal; means for amplifying said FM input signal; a first balanced bifilar transformer for receiving said amplified signal at its primary windings and for issuing an output signal at its secondary windings; a pair of transistors arranged in balanced push-pull relation for receiving said output signal from the secondary windings of said first balanced bifilar transformer; a second balanced bifilar transformer connected so that its primary windings are fed by said pair of transistors and means connecting the secondary windings of said transformer to drive said balanced transmission line phase bridge discrimina- 6. A balanced push-pull FM detector, including a singleended driver amplifier, an input transformer including a single ended primary winding, a balanced push-pull secondary winding having two secondary halves, said secondary halves being bifilarly wound with respect to each other and with respect to said primary winding, means for only AC grounding the junction of said secondary halves, means for only AC grounding one end only of said primary winding, and a push-pull amplifier configuration having two transistors connected in pushpull, each of said transistors having a base connected to a different one of the ungrounded terminals of said secondary winding, an output bifilar transformer for said two transformers having a balanced bifilarly wound primary winding and a single ended secondary winding bifilarly wound with said balanced primary winding, and a balanced transmission line phase bridge frequency discriminator connected to be driven by said secondary winding.

7. In a frequency discrimination system, a balanced transmission line-bridge-type frequency modulation discriminator, an amplifier connected to drive said discriminator, said amplifier being free of harmonic response when driven by a wide band frequency modulated signal.

i t i 

1. A method for achieving distortionless response of a balanced transmission line phase bridge discriminator, comprising the steps of: receiving an FM input signal; so amplifying the FM input signal as to produce essentially zero even-order harmonics in the amplified output and feeding the so amplified output to a balanced transmission line phase bridge discriminator.
 2. A circuit for providing distortionless response of a balanced transmission line phase bridge discriminator, comprising: means for receiving an FM input signal; amplifier means for amplifying the FM input signal; filter means for receiving from said amplifier the amplified FM input signal, said filter being arranged and adapted to issue a signal which is a replica of the FM input signal devoid of harmonics; and means for feeding said replica to the input of said balanced transmission line bridge discriminator.
 3. A balanced transmission line phase bridge discriminator system, comprising: means for receiving an FM input signal; means for amplifying said FM input signal; push-pull power amplifier means for receiving said amplified input signal, said push-pull amplifier means being arranged and adapted to transfer said amplified signal without even-order harmonics; and means for feeding the resultant signal to said balanced transmission line bridge discriminator.
 4. The circuit as described in claim 3, wherein said means for controlling the frequency response rolloff includes at least one balanced bifilar transformer connected as the output circuit of said push-pull power amplifier.
 5. A circuit for driving a balanced transmission line phase bridge discriminator, comprising: means for receiving an FM input signal; means for amplifying said FM input signal; a first balanced bifilar transformer for receiving said amplified signal at its primary windings and for issuing an output signal at its secondary windings; a pair of transistors arranged in balanced push-pull relation for receiving said output signal from the secondary windings of said first balanced bifilar transformer; a second balanced bifilar transformer connected so that its primary windings are fed by said pair of transistors and means connecting the secondary windings of said transformer to drive said balanced transmission line phase bridge discriminator.
 6. A balanced push-pull FM detector, including a single-ended driver amplifier, an input transformer including a single ended primary winding, a balanced push-pull secondary winding having two secondary halves, said secondary halves being bifilarly wound with respect to each other and with respect to said primary winding, means for only AC grounding the junction of said secondary halves, means for only AC grounding one end only of said primary winding, and a push-pull amplifier configuration having two transistors connected in push-pull, each of said transistors having a base connected to a different one of the ungrounded terminals of said secondary winding, an output bifilar transformer for said two transformers having a balanced bifilarly wound primary winding and a single ended secondary winding bifilarly wound with said balanced primary winding, and a balanced transmission line phase bridge frequency discriminator connected to be driven by said secondary winding.
 7. In a frequency discrimination system, a balanced transmission line-bridge-type frequency modulation discriminator, an amplifier connected to drive said discriminator, said amplifier being free of harmonic response when driven by a wide band frequency modulated signal. 